A computed tomography (CT) scanner includes a rotating portion rotatably supported by a stationary portion. The rotating portion supports an x-ray tube, which emits radiation that traverses an examination region and an object or a subject therein, and a detector array that detects radiation traversing the examination region and generates projection data indicative of the detected radiation. A subject support supports the object or subject in the examination region. A reconstructor reconstructs the projection data and generates volumetric image data indicative of the portion of the object or subject in the examination region. One or more images can be generated based on the volumetric image data.
The subject support generally includes a base, which is affixed to the floor of the examination room and configured to move vertically with respect to the floor, and a tabletop, which is affixed to the base and is configured to translate horizontally, with respect to the base, in and out of the examination region. Both axial and perfusion scans require frequent tabletop translation; an axial scan is an imaging mode in which the tabletop moves the object or subject to each axial position for scanning, but does not move the object or subject during scanning, and a perfusion scan is an imaging mode in which the tabletop cycles the object or subject back and forth in the examination region during scanning.
Traditionally, tabletop translation for both axial and perfusion scans is accomplished using a trapezoidal or an s-curve motion algorithm. FIG. 1 illustrates an example s-curve motion algorithm for axial (step and shoot) scanning over three steps. In FIG. 1(A), a y-axis 102 represents tabletop position, in FIG. 1(B), a y-axis 104 represents tabletop velocity, and in FIG. 1(C), a y-axis 106 represents tabletop acceleration. In all three figures, an x-axis 108 represents time. As shown in FIG. 1(C), tabletop acceleration and deceleration for the s-curve motion algorithm includes abrupt variable ramp up and ramp down, for each step.
FIGS. 2 and 3 respectively illustrate example trapezoidal and s-curve motion algorithms for perfusion (cyclic) scanning In FIGS. 2(A) and 3(A), the y-axis 102 represents tabletop position, in FIGS. 2(B) and 3(B), the y-axis 104 represents tabletop velocity, and in FIGS. 2(C) and 3(C), the y-axis 106 represents tabletop acceleration, and the x-axis 108 in all six figures represents time. FIGS. 2(C) and 3(C) respectively show tabletop acceleration and deceleration with constant ramp up and ramp down and abrupt variable ramp up and ramp down, during each cycle.
The acceleration and deceleration profiles of the trapezoidal and s-curve motion algorithms of FIGS. 1-3 generally will result in tabletop vibration and motion of internal organs and tissue of the patient being scanned. More particularly, the tabletop is a lightly damped steel and composite structure with one or multiple resonant frequencies. Upon the moment of trapezoidal or s-curve point to point motion in the horizontal direction, the tabletop resonance can be aroused by the resonant component of the primary motion, resulting in undesired secondary motion or diving board vibration in the vertical direction. High acceleration and jerk due to the primary motion, when used cyclically in perfusion scans, may cause excessive human organ motion, resulting in an unpleasant patient experience.
Unfortunately, with axial scans, as the tabletop moving and settling time between scans becomes shorter and the intermitted travel becomes longer with wider coverage, motion acceleration and jerk will become greater, and, with perfusion scans, as the coverage becomes wider and the cycle time becomes shorter, perfusion scans can introduce greater secondary vibration. The secondary motion can be mitigated by use of a rigid based structure. However, with the conventional trapezoidal and s-curve algorithms, even with a stiff base structure, the amount of human organ motion and undesirable patient feeling generally cannot be mitigated.